Well, I think we're talking about fictional scenarios here. If Lebron ever gone through the tennis route as a kid, he wouldn't need to reach his 6'8" height as he is in basketball. My main argument is that how Lebron is, he is clearly behind at least Federer in terms of overall ability - tennis is harder to play than basketball (fact). Again jumping to consistently get layups at his athletic nature requires practice, but isn't that hard of an aspect for the entire general population to do. I have no problem labeling him one of the best or the best athletic player ever, but it's hard to pinpoint how much technical/talent he has since basketball as a natural sport requires a ton of athleticism and not a ton of talent.
Absolutely yes he could because players from different sports have switched to baseball and have hit major league pitches - Jordan and Tebow. Hell even pylon Beauchemin hit a freakin' homerun - you cannot say the same thing about Mike Trout sniping on an NHL goaler. Neither of them were good, true, but baseball is one sport where other major athletes can come in and at least be passable. That's what lowers baseball for me, even though the mentality between the pitcher and batter is probably the toughest event in activities'.